(Dialog) Component-less alert and confirm dialogs

#### Feature Description

My proposal is to create one or two methods on the `MatDialog` provider/controller to allow component-less alert and confirm dialogs, for example:

// Alert dialog
this.dialog.alert({
title: 'Forbidden',
message: 'The resource you are trying to access is forbidden.',
okButton: 'OK'
})

// Confirm dialog
this.dialog.confirm({
title: 'Server error',
message: 'The server is currently in maintenance.',
confirmButton: 'Retry',
cancelButton: 'Cancel'
})

Alternatively, both `alert` and `confirm` could be on a single method:

this.dialog.alert({
title: 'The title',
message: 'The message',
confirmButton: 'OK', // Optional, only for confirm
dismissButton: 'Dismiss'
}).afterClose().subscribe((result: boolean) => console.log); // True if confirm, false if dismiss.

#### Use Case

Currently, we have to create a component for every dialog, event the simplest dialog. This is painful and useless in most cases. Other alternatives (what I do) is to create a `AlertService` that does it for me. But makes no sense maintain it if it could be a simple feature of `MatDialog`

Beginne mit der Prüfung des bestehenden MatDialog provider/controller und des aktuellen component-based dialog flow. Vergleiche die vorgeschlagenen alert- und confirm-APIs, einschließlich der button labels und des boolean close result. Als abgeschlossen gilt die Arbeit, wenn das component-less-Verhalten und die API-Auswahl für beide dialog outcomes definiert und implementiert sind.

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